The proposed investigation seeks to examine the use of "club drugs" among male and female young adults (ages 18-25) in New York City (NYC). We will undertake a longitudinal, prospective study, using qualitative and quantitative methods, guided by the following three aims: (1) To determine patterns of use, individual differences in use, and changes in club drug use among heterosexual and gay/bisexual male and female young adults (ages 18-25) in NYC over the course of one year; (2) To determine the extent to which antecedent person factors, social context factors, and constructs associated with the Theory of Reasoned Action (TRA) explain differences in year-long club drug use trajectories of young adults in NYC; and (3) To better understand how patterns of club drug use impact sexual risk taking behaviors among young adults in NYC. Our work will be guided by the TRA in order to determine how antecedents of club drug use influence the use of these substances, and how the interaction of these antecedents with actual use of club drugs leads to sexual risk taking behaviors. Our work will focus on six club drugs that our pilot work has shown to be used among young adults in NYC who frequent dance clubs: Cocaine, LSD, GFB, Ketamine, MDMA, and Methamphetamine. We will recruit 480 ethnically-diverse young adults into our study (stratified by gender and sexual orientation) using probability-based targeted sampling and conduct assessments of the antecedents, club drug use, and sexual risk-taking behaviors four times over the course of one year. Data analyses will utilize univariate and multivariate growth curve modeling to determine the relationships between these factors as a means of understanding patterns of club drug use and of evaluating the use of the TRA to explain club drug use. Quantitative assessments (administered via Audio-CASI) will be complemented by our use of qualitative techniques to provide us with episode-specific data to help realize the relationships that we are assessing as well as to further understand contextual aspects of club drug use (e.g., use of drugs in and out of dance club venues, reasons for using certain drugs, reasons for mixing multiple drugs, social context of club drug use, use of club drugs in conjunction with sexual behaviors). Mixed-methods strategies will be used to examine the qualitative and quantitative data together in order to further understand the data and potential implications for intervention development.